:mod:`dbm` --- Interfaces to Unix "databases" ============================================= .. module:: dbm :synopsis: Interfaces to various Unix "database" formats. :mod:`dbm` is a generic interface to variants of the DBM database --- :mod:`dbm.bsd` (requires :mod:`bsddb`), :mod:`dbm.gnu`, or :mod:`dbm.ndbm`. If none of these modules is installed, the slow-but-simple implementation in module :mod:`dbm.dumb` will be used. .. exception:: error A tuple containing the exceptions that can be raised by each of the supported modules, with a unique exception also named :exc:`dbm.error` as the first item --- the latter is used when :exc:`dbm.error` is raised. .. function:: whichdb(filename) This functionattempts to guess which of the several simple database modules available --- :mod:`dbm.bsd`, :mod:`dbm.gnu`, :mod:`dbm.ndbm` or :mod:`dbm.dumb` --- should be used to open a given file. Returns one of the following values: ``None`` if the file can't be opened because it's unreadable or doesn't exist; the empty string (``''``) if the file's format can't be guessed; or a string containing the required module name, such as ``'dbm.ndbm'`` or ``'dbm.gnu'``. .. function:: open(filename[, flag[, mode]]) Open the database file *filename* and return a corresponding object. If the database file already exists, the :func:`whichdb` function is used to determine its type and the appropriate module is used; if it does not exist, the first module listed above that can be imported is used. The optional *flag* argument can be ``'r'`` to open an existing database for reading only, ``'w'`` to open an existing database for reading and writing, ``'c'`` to create the database if it doesn't exist, or ``'n'``, which will always create a new empty database. If not specified, the default value is ``'r'``. The optional *mode* argument is the Unix mode of the file, used only when the database has to be created. It defaults to octal ``0o666`` (and will be modified by the prevailing umask). The object returned by :func:`open` supports most of the same functionality as dictionaries; keys and their corresponding values can be stored, retrieved, and deleted, and the :keyword:`in` operator and the :meth:`keys` method are available. Keys and values must always be strings. The following example records some hostnames and a corresponding title, and then prints out the contents of the database:: import dbm # Open database, creating it if necessary. db = dbm.open('cache', 'c') # Record some values db['www.python.org'] = 'Python Website' db['www.cnn.com'] = 'Cable News Network' # Loop through contents. Other dictionary methods # such as .keys(), .values() also work. for k, v in db.iteritems(): print(k, '\t', v) # Storing a non-string key or value will raise an exception (most # likely a TypeError). db['www.yahoo.com'] = 4 # Close when done. db.close() .. seealso:: Module :mod:`shelve` Persistence module which stores non-string data. The individual submodules are described in the following sections. :mod:`dbm.bsd` --- DBM-style interface to the BSD database library ------------------------------------------------------------------ .. module:: dbm.bsd :synopsis: DBM-style interface to the BSD database library. .. sectionauthor:: Fred L. Drake, Jr. .. index:: module: bsddb The :mod:`dbm.bsd` module provides a function to open databases using the BSD ``db`` library. This module mirrors the interface of the other Python database modules that provide access to DBM-style databases. The :mod:`bsddb` module is required to use :mod:`dbm.bsd`. .. exception:: error Exception raised on database errors other than :exc:`KeyError`. It is a synonym for :exc:`bsddb.error`. .. function:: open(path[, flag[, mode]]) Open a ``db`` database and return the database object. The *path* argument is the name of the database file. The *flag* argument can be: +---------+-------------------------------------------+ | Value | Meaning | +=========+===========================================+ | ``'r'`` | Open existing database for reading only | | | (default) | +---------+-------------------------------------------+ | ``'w'`` | Open existing database for reading and | | | writing | +---------+-------------------------------------------+ | ``'c'`` | Open database for reading and writing, | | | creating it if it doesn't exist | +---------+-------------------------------------------+ | ``'n'`` | Always create a new, empty database, open | | | for reading and writing | +---------+-------------------------------------------+ For platforms on which the BSD ``db`` library supports locking, an ``'l'`` can be appended to indicate that locking should be used. The optional *mode* parameter is used to indicate the Unix permission bits that should be set if a new database must be created; this will be masked by the current umask value for the process. The database objects returned by :func:`open` provide the methods common to all the DBM-style databases and mapping objects. The following methods are available in addition to the standard methods: .. method:: dbhash.first() It's possible to loop over every key/value pair in the database using this method and the :meth:`next` method. The traversal is ordered by the databases internal hash values, and won't be sorted by the key values. This method returns the starting key. .. method:: dbhash.last() Return the last key/value pair in a database traversal. This may be used to begin a reverse-order traversal; see :meth:`previous`. .. method:: dbhash.next() Returns the key next key/value pair in a database traversal. The following code prints every key in the database ``db``, without having to create a list in memory that contains them all:: print(db.first()) for i in range(1, len(db)): print(db.next()) .. method:: dbhash.previous() Returns the previous key/value pair in a forward-traversal of the database. In conjunction with :meth:`last`, this may be used to implement a reverse-order traversal. .. method:: dbhash.sync() This method forces any unwritten data to be written to the disk. :mod:`dbm.gnu` --- GNU's reinterpretation of dbm ------------------------------------------------ .. module:: dbm.gnu :platform: Unix :synopsis: GNU's reinterpretation of dbm. This module is quite similar to the :mod:`dbm` module, but uses the GNU library ``gdbm`` instead to provide some additional functionality. Please note that the file formats created by ``gdbm`` and ``dbm`` are incompatible. The :mod:`dbm.gnu` module provides an interface to the GNU DBM library. ``gdbm`` objects behave like mappings (dictionaries), except that keys and values are always strings. Printing a :mod:`dbm.gnu` object doesn't print the keys and values, and the :meth:`items` and :meth:`values` methods are not supported. .. exception:: error Raised on ``gdbm``\ -specific errors, such as I/O errors. :exc:`KeyError` is raised for general mapping errors like specifying an incorrect key. .. function:: open(filename, [flag, [mode]]) Open a ``gdbm`` database and return a :class:`gdbm` object. The *filename* argument is the name of the database file. The optional *flag* argument can be: +---------+-------------------------------------------+ | Value | Meaning | +=========+===========================================+ | ``'r'`` | Open existing database for reading only | | | (default) | +---------+-------------------------------------------+ | ``'w'`` | Open existing database for reading and | | | writing | +---------+-------------------------------------------+ | ``'c'`` | Open database for reading and writing, | | | creating it if it doesn't exist | +---------+-------------------------------------------+ | ``'n'`` | Always create a new, empty database, open | | | for reading and writing | +---------+-------------------------------------------+ The following additional characters may be appended to the flag to control how the database is opened: +---------+--------------------------------------------+ | Value | Meaning | +=========+============================================+ | ``'f'`` | Open the database in fast mode. Writes | | | to the database will not be synchronized. | +---------+--------------------------------------------+ | ``'s'`` | Synchronized mode. This will cause changes | | | to the database to be immediately written | | | to the file. | +---------+--------------------------------------------+ | ``'u'`` | Do not lock database. | +---------+--------------------------------------------+ Not all flags are valid for all versions of ``gdbm``. The module constant :const:`open_flags` is a string of supported flag characters. The exception :exc:`error` is raised if an invalid flag is specified. The optional *mode* argument is the Unix mode of the file, used only when the database has to be created. It defaults to octal ``0o666``. In addition to the dictionary-like methods, ``gdbm`` objects have the following methods: .. method:: gdbm.firstkey() It's possible to loop over every key in the database using this method and the :meth:`nextkey` method. The traversal is ordered by ``gdbm``'s internal hash values, and won't be sorted by the key values. This method returns the starting key. .. method:: gdbm.nextkey(key) Returns the key that follows *key* in the traversal. The following code prints every key in the database ``db``, without having to create a list in memory that contains them all:: k = db.firstkey() while k != None: print(k) k = db.nextkey(k) .. method:: gdbm.reorganize() If you have carried out a lot of deletions and would like to shrink the space used by the ``gdbm`` file, this routine will reorganize the database. ``gdbm`` will not shorten the length of a database file except by using this reorganization; otherwise, deleted file space will be kept and reused as new (key, value) pairs are added. .. method:: gdbm.sync() When the database has been opened in fast mode, this method forces any unwritten data to be written to the disk. :mod:`dbm.ndbm` --- Interface based on ndbm ------------------------------------------- .. module:: dbm.ndbm :platform: Unix :synopsis: The standard "database" interface, based on ndbm. The :mod:`dbm.ndbm` module provides an interface to the Unix "(n)dbm" library. Dbm objects behave like mappings (dictionaries), except that keys and values are always strings. Printing a dbm object doesn't print the keys and values, and the :meth:`items` and :meth:`values` methods are not supported. This module can be used with the "classic" ndbm interface, the BSD DB compatibility interface, or the GNU GDBM compatibility interface. On Unix, the :program:`configure` script will attempt to locate the appropriate header file to simplify building this module. .. exception:: error Raised on dbm-specific errors, such as I/O errors. :exc:`KeyError` is raised for general mapping errors like specifying an incorrect key. .. data:: library Name of the ``ndbm`` implementation library used. .. function:: open(filename[, flag[, mode]]) Open a dbm database and return a dbm object. The *filename* argument is the name of the database file (without the :file:`.dir` or :file:`.pag` extensions; note that the BSD DB implementation of the interface will append the extension :file:`.db` and only create one file). The optional *flag* argument must be one of these values: +---------+-------------------------------------------+ | Value | Meaning | +=========+===========================================+ | ``'r'`` | Open existing database for reading only | | | (default) | +---------+-------------------------------------------+ | ``'w'`` | Open existing database for reading and | | | writing | +---------+-------------------------------------------+ | ``'c'`` | Open database for reading and writing, | | | creating it if it doesn't exist | +---------+-------------------------------------------+ | ``'n'`` | Always create a new, empty database, open | | | for reading and writing | +---------+-------------------------------------------+ The optional *mode* argument is the Unix mode of the file, used only when the database has to be created. It defaults to octal ``0o666`` (and will be modified by the prevailing umask). :mod:`dbm.dumb` --- Portable DBM implementation ----------------------------------------------- .. module:: dbm.dumb :synopsis: Portable implementation of the simple DBM interface. .. index:: single: databases .. note:: The :mod:`dbm.dumb` module is intended as a last resort fallback for the :mod:`dbm` module when no more robust module is available. The :mod:`dbm.dumb` module is not written for speed and is not nearly as heavily used as the other database modules. The :mod:`dbm.dumb` module provides a persistent dictionary-like interface which is written entirely in Python. Unlike other modules such as :mod:`gdbm` and :mod:`bsddb`, no external library is required. As with other persistent mappings, the keys and values must always be strings. The module defines the following: .. exception:: error Raised on dbm.dumb-specific errors, such as I/O errors. :exc:`KeyError` is raised for general mapping errors like specifying an incorrect key. .. function:: open(filename[, flag[, mode]]) Open a dumbdbm database and return a dumbdbm object. The *filename* argument is the basename of the database file (without any specific extensions). When a dumbdbm database is created, files with :file:`.dat` and :file:`.dir` extensions are created. The optional *flag* argument is currently ignored; the database is always opened for update, and will be created if it does not exist. The optional *mode* argument is the Unix mode of the file, used only when the database has to be created. It defaults to octal ``0o666`` (and will be modified by the prevailing umask). In addition to the methods provided by the :class:`collections.MutableMapping` class, :class:`dumbdbm` objects provide the following method: .. method:: dumbdbm.sync() Synchronize the on-disk directory and data files. This method is called by the :meth:`Shelve.sync` method.